Harmonic termination circuit for medium bandwidth microwave power amplifiers

Abstract

A compact amplifier output bias circuit is used as a broadband harmonic termination. The bias circuit is adapted as a harmonic termination circuit to produce an effective low impedance at the signal harmonic frequencies while having the capability of supplying DC power to the amplifier stage, optionally, if needed. A pi network is coupled to an active device output through a predetermined length of transmission line tuned for optimum power added efficiency in the frequency band of operation and provides a low impedance at frequency bands above the frequency band of operation while allowing DC bias to be appliable to the active device output.

1. A method of broadband termination of amplifier harmonics from an active device, comprising:
 configuring a pi network providing a low impedance at frequency bands above a frequency band of operation while allowing DC bias to be appliable to the active device output; and 
 coupling the pi network to the active device output through a predetermined length of transmission line tuned for optimum power added efficiency in the frequency band of operation, 
 wherein configuring the pi network includes:
 coupling a low pass filter to the predetermined length of transmission line, the low pass filter having an RF choke and a by-pass capacitor; and 
 coupling a capacitive stub in parallel with the low pass filter, the capacitive stub being valued to resonate with the RF choke to produce a high impedance at a desired frequency and a low impedance to at least a third harmonic of the desired frequency. 
 
 
   
   
     2. A broadband harmonic termination apparatus for an active device output comprising:
 a pi network coupleable to the active device output through a predetermined length of transmission line tuned for optimum power added efficiency in a frequency band of operation, the pi network providing a low impedance at frequency bands above the frequency band of operation while allowing DC bias to be applied to the active device output, 
 wherein the pi network includes:
 a low pass filter coupled to an active device drain through the predetermined length of transmission line, the low pass filter having an RF choke and a by-pass capacitor; and 
 a capacitive stub coupled in parallel with the low pass filter, the capacitive stub being valued to resonate with the RF choke to produce a high impedance at a desired frequency and a low impedance to at least a third harmonic of the desired frequency. 
 
 
   
   
     3. A method of power combining having broadband termination of amplifier harmonics, comprising:
 providing a power combining network having a plurality of input ports and a power combined output port; 
 coupling respective amplifiers to the input ports, each amplifier having an active device output; 
 configuring a pi network providing a low impedance at frequency bands above a frequency band of operation while allowing DC bias to be appliable to the active device output and coupling a respective pi network to each active device output through a respective predetermined length of transmission line tuned for optimum power added efficiency in the frequency band of operation, 
 wherein configuring the pi network includes:
 coupling a low pass filter to the predetermined length of transmission line, the low pass filter having an RF choke and a by-pass capacitor; and 
 coupling a capacitive stub in parallel with the low pass filter, the capacitive stub being valued to resonate with the RF choke to produce a high impedance at a desired frequency and a low impedance to at least a third harmonic of the desired frequency. 
 
 
   
   
     4. A power combiner having broadband termination of amplifier harmonics, comprising:
 a power combining network having a plurality of input ports and a power combined output port; 
 respective amplifiers coupled to the input ports, each amplifier having an active device output; and 
 a pi network configured to provide a low impedance at frequency bands above a frequency band of operation while allowing DC bias to be appliable to the active device output, wherein a respective pi network is coupled to each active device output through a respective predetermined length of transmission line tuned for optimum power added efficiency in the frequency band of operation, 
 wherein the pi network includes:
 a low pass filter coupled to the predetermined length of transmission line, the low pass filter having an RF choke and a by-pass capacitor; and 
 a capacitive stab coupled in parallel with the low pass filter, the capacitive stub being valued to resonate with the RF choke to produce a high impedance at a desired frequency and a low impedance to at least a second harmonic of the desired frequency.